Adult T Cell Leukemia/Lymphoma (ATLL) Is a Malignant Counterpart of Regulatory T Cells (Treg).

Adult T cell leukemia/lymphoma (ATLL) is a lymphoproliferative disorder caused by a retrovirus, human T-lymphotropic virus type 1 (HTLV-1). ATLL is subclassified into four subtypes, smoldering, chronic, acute, and lymphoma. The acute type progresses rapidly and is usually resistant to conventional chemotherapy. In contrast, the chronic type shows an indolent clinical course and the patients survive for several years, even without chemotherapy. Irrespective of the subtypes, however, ATLL patients are in a severely immune-suppressed condition and can easily acquire opportunistic infections such as Pneumocystis Carinii pneumonitis. Suppression of cell-mediated immunity has also been reported in HTLV-1 carriers. Although ATLL cells show the activated helper/inducer T-cell phenotypes, CD4+ and CD25+, they exhibit strong immune-suppressive activity in vitro. The recent notion of CD4+ CD25+ regulatory T cells (Treg) prompted us to investigate the origin of ATLL cells from the standpoint of Treg. Forkhead/winged helix transcription factor (Foxp3) is a functional marker of Treg, which plays a central role in their generation. There are other marker molecules for Treg, including glucocorticoid-induced TNFR family-related protein (GITR) and the chemokine receptors CCR4 and CCR8.

In the present study, we examined primary ATLL cells from 48 patients: 36 patients with acute type and 12 patients with chronic type. We also examined ATLL cell lines, HTLV-1-infected T-cell lines and peripheral blood mononuclear cells (PBMC) from healthy adults as control cells. We used RT-PCR for detection of Foxp3, GITR, CCR4, and CCR8 mRNA expression. Foxp3 and/or GITR mRNA were detected in over 90% of the patients, and 50% of the patients expressed both. There was no difference between subtypes. In contrast, Foxp3 and GITR mRNA were scarcely detected in the PBMC from healthy adults. Furthermore, we confirmed GITR expression at the protein level by flow cytometry. CCR4 and CCR8 mRNA were also detected in almost all ATLL samples, at significantly higher levels than in the normal PBMC. Corresponding to the results of the primary cells, ATLL cell lines and HTLV-1-infected T-cell lines also expressed GITR mRNA, although HTLV-1-negative cell lines, Jurkat and Molt4, completely lack it. Next, we examined whether GITR affects ATLL cell proliferation using a GITR- expressing IL-2-dependent ATLL cell line, KK1. We found that GITR ligand induced proliferation of KK1 cells in an IL-2-negative condition. Thus, these results indicate the Treg origin of ATLL cells and show that GITR expression is possibly involved in the development of ATLL.